They're so cheap and the thickness on these is perfect for training.

Probably a bit too soft

do you guys think it would be possible to grab some gaffers tape and say 5 or 6 of these and stitch them together into something usuable?

For the price of 5-6 of these mats combined with the effort of getting them somehow together in the long term you could probably buy better options.

For less than double the price of a single mat you could get something like this on Amazon and it is in stock.

Puzzle mats already have the locking mechanism built into them and if they still slip a bit, you could throw a cheap rubber mat e.g. a horse stall mat under them with the added bonus of minimal extra padding.