A recommendation I would make...
This style of workbench has lots of merit! I think I would make each of the two lengthwise pipes become two clamps - one on each end, for greater flexibility; because on pipe clamps the pipe isn't moving. With oppositely facing (pipe clamp, movable) collars, it could also serve as a spreader. This kind of clamp will best work like he had it in the video - where someone has room to work on all four edges. IMHO, an adjustment to methodology of support for the removable inserts is required; because without a continuous "edge ledge" - there will be uplift of the (insert's) edge opposite where downward pressure is applied. In the video, I gathered that his inserts are only supported by the longitudinal pipes.
[...and a nebulous point also, there is a mathematical progression which enables sums to be achieved of any length in multiples of the smallest (part) increment. In this case one could determine the amount of stop-block movement possible (= approximate length of pipe clamp spindle threads) and use that as the basic increment.]
Here's an example of this progression. Let's use an even integer by saying that the grip range of each end spindle is 2". Make interior filler parts of numbers doubled from 2"... First piece would be 2", next would be 4" (double 2"), next would be 8" (double 4"), next would be 16" (double 8"), next would be 32" (double 16"), etc. With parts in this list of lengths, one can create an opening of any length between 2" and [max], in 2" multiples.
Please do not take my comments as criticism! These are only suggestions I would make to better fit my needs. Inventing and/or improving products is what I do to make my living. The bench - exactly as on his video will work perfectly fine.
OPG3 Tweak everything!