From the archaeological evidence, we did found nails driven into skeletons by the Romans which proof that the Romans did nail people onto wooden structures or trees, but we do not know exactly how the structure looked on which Jesus was nailed.

Both John 19:17-18 and the Jewish historian Josephus, who lived in the first century, used the Greek word stauros which means "a stake or an implement of capital punishment". In older Greek texts, "stauros" means "pole".

However, because many translators commonly translated "stauros" as "cross" in the Bible, many were lead to believe that "stauros" means "cross", but that is circular reasoning and does not proof anything.

The Septuagint is the Greek translation of the Hebrew Bible, written by 72 Jewish scholars at the request of Ptolemy II Philadelphus, king of Egypt. This translation does not contain any references to "cross" symbols or that the Christ will be "crucified". It only states that Christ will be:

pierced for our transgressions; ... crushed for our iniquities; -- Isaiah 53:5 (ESV)

Even Peter said:

He himself bore our sins in his body on "xylon", that we might die to sin and live to righteousness. -- 1 Peter 2:24 (ESV)

"xylon" is the Greek word for "wood" or "timber" which could mean "tree" (as translated in the ESV, Berean, KJV, NKJV, ASV, ERV, ISV, Majority, NET, Webster, WEB, YLT, etc.), but could also refer to any general "wooden" structure (as translated in Godbey, Smith) which could be a "cross" (as translated in NIV, NLT, NASB, AMP, CEV, GNT, NRSV, etc.)

In conclusion, the shape of the structure is not as important as the church had lead to believe Christians. If "the cross" was important, then at least one prophet would have mentioned a cross somewhere.