To stick my oar in too :lol: I have never had to strip an UGF (like Alan the only time I did so was when moving the tank) and I have had them running 7 or 8 years non stop. Everyone knows I am very lax in my water change and maintenance routines but never had a problem.
As stated before I believe, the best filter for the job depends on which fish you want to keep.
As to the original question - sorry fins, I have never tried to make an UGF so can't help you there. They are so cheap to buy and you can lock them together to cover most of the base. The do not have to fit snugly all round the sides either.