Some stick down the mylar tape, then when cured and firm (24-hr), use a very sharp knife to slice down the centre-line.
Risk is (a) cut is not straight (it's ever so easy for the knife to 'wander', even when you think you have a hard-edge to follow !) so offering-up soft-spots for the c/b to catch on and bend, or (b) any pause in the cutting produces an imperfect slice with a small nick that again can catch the c/b.
Using over-long tape pinned down onto a long firm plank, cut down the centreline of the tape, then stick tape (shortened to correct length) with original (outer) edges meeting at the centre, or even overlapping by 1/8 inch (but no more).
I used to do the 'alternative' very successfully on Fireballs, but have recently become lazy, accepted the risks and employed the "stick then cut-down-the-middle" technique.
For both, a small "V" at the stern end helps the slot empty by venturi effect, just like a self-bailer.
Also concur the importance of a ~3cm length of non-stuck at the front end to allow flexibility when the c/b is fully down, and a short transverse sacrificial piece to ensure leading edge stays down (for this I use the brilliant Conservatory Roof Repair Tape (Mylar) described elsewhere).