Justin Martyr's First Apology, Chapter LV.-Symbols of the Cross.
http://www.ccel.org/fathers2/ANF-01/anf01-46.htm
"For the one beam is placed upright, from which the highest extremity is raised up into a horn, when the other beam is fitted on to it, and the ends appear on both sides as horns joined on to the one horn."
Justin's "Dialogue With Trypho", Chap XC in ANF, p. 245
In the first century B.C. Dionysius of Halicarnassus described the practice of tying the patibulum across the victims back:
"A Roman citizen of no obscure station, having ordered one of his slaves to be put to death, delivered him to his fellow-slaves to be led away, and in order that his punishment might be witnessed by all, directed them to drag him through the Forum and every other conspicuous part of the city as they whipped him, and that he should go ahead of the procession which the Romans were at that time conducting in honour of the god. The men ordered to lead the slave to his punishment, having stretched out both his arms and fastened them to a piece of wood which extended across his breast and shoulders as far as his wrists, followed him, tearing his naked body with whips."
(Roman Antiquities, 7.69.1-2)
Seneca lived from 4 B.C. - A.D. 65, was a Roman and wrote the following:
*** refigere se crucibus conentur, in quas unusquisque vestrum clavos suos ipse adigit, ad supplicium tamen acti stipitibus singulis pendent; hi, qui in se ipsi animum advertunt, quot cupiditatibus tot crucibus distrahuntur. At maledici et in alienam contumeliam venusti sunt. Crederem illis hoc vacare, nisi quidam ex patibulo suo spectatores conspuerent! "Though they strive to release themselves from their crosses---those crosses to which each one of you nails himself with his own hand--yet they, when brought to punishment hang each one on a single stipes; but these others who bring upon themselves their own punishment are stretched upon as many crosses as they had desires. Yet they are slanderous and witty in heaping insult on others. I might believe that they were free to do so, did not some of them spit upon spectators from their own patibulum!" (De Vita Beata, 19.3).
....alium in cruce membra distendere.... "another to have his limbs stretched upon the crux" (De Ira, 1.2.2).
Video istic cruces non unius quidem generis sed aliter ab aliis fabricatas: capite quidam conversos in terram suspendere, alii per obscena stipitem egerunt, alii brachia patibulo explicuerunt. "Yonder I see crosses, not indeed of a single kind, but differently contrived by different peoples; some hang their victims with head toward the ground, some impale their private parts, others stretch out their arms on a patibulum" (De Consolatione, 20.3).
Contempissimum putarem, si vivere vellet usque ad crucem....Est tanti vulnus suum premere et patibulo pendere districtum.... Invenitur, qui velit adactus ad illud infelix lignum, iam debilis, iam pravus et in foedum scapularum ac pectoris tuber elisus, cui multae moriendi causae etiam citra crucem fuerant, trahere animam tot tormenta tracturam? "I should deem him most despicable had he wished to live up to the very time of crucifixion....Is it worth while to weigh down upon one's own wound, and hang impaled upon a patibulum?....Can any man be found willing to be fastened to the accursed tree, long sickly, already deformed, swelling with ugly tumours on chest and shoulders, and draw the breath of life amid long drawn-out agony? I think he would have many excuses for dying even before mounting the crux!" (Epistle, 101.10-14).
Cogita hoc loco carcerem et cruces et eculeos et uncum et adactum per medium hominem, qui per os emergeret, stipitem. "Picture to yourself under this head the prison, the crux, the rack, the hook, and the stake which they drive straight through a man until it protrudes from his throat" (Epistle, 14.5).
....sive extendendae per patibulum manus "....or his hands to be extended on a patibulum" (Fragmenta, 124; cf. Lactantius, Divinis Institutionibus, 6.17).
There is also testimony about the form of the cross by early non-Christian writers. The Greek writer Lukianos (c. 120-180 AD) wrote that the letter T had received its "evil meaning" because of the "evil instrument tyrants put up to hang people upon them. (Lukianos in "Iudicium Vocalium 12", in Crucifixion by Martin Hengel, Fortress Press, 1982, pp. 8,9)
Artemidorus lived in the 2nd century AD during the reigns of Hadrian and the Antonines. In his five-volume work Oneirocritica (The Interpretation of Dreams) he also compares the stauros to a ship:
"Being crucified is auspicious for all seafarers. For the stauros, like a ship, is made of wood and nails, and the ship's mast resembles a stauros."
Artemidorus, Oneirocritica, 2:53
We have evidence from the early Bible manuscripts themselves. The manuscripts P66 and P75 are traditionally dated around AD 200, but may be from as early as the last part of the first century. (See BIBLICA , Vol. 69:2, 1988; which dates the much related P46 this early, and preliminary information from Professor George Howard by letter stated P75 and P66 are "not far behind" in date.)
In P75 the word "stauros" is changed so the T and R together depict a cross with a person on in three places where it occurs, and P66 put a cross into the word "stauros."