Use one or two strips of indoor double sided carpet tape, down the centre of the reflector. the next part is a two person job: Hold the mylar over the upside down reflector, and start at one end, stick it down to the tape - keep working along until the entire length of the mylar is stuck down. for sticking the lateral parts of the mylar down, you just need
three of four 2" lengths of tape, on each side. this is how
i managed to do a perfect job of fitting mylar inside the fixtures. consider overdriving the fixtures, if you have a budget for a couple more ballasts.

cannabisgrower,

Please describe the overdrive process.

John

there is a very long thread down below on this forum showing how to do it. with my HD fixture, it's an instant start, and it has two blue wires, each going to one end of each of the two lamps. the pins of the lamp are connected together (shorted), either by a wire, or internally in the little lamp holder thingy. this lamp is started by a burst of high voltage (instead of warming up the heater filament, which is called rapid start). That's how it come. NOW FOR OVERDRIVING: i connected both blue wires together, and have the two of them supply JUST ONE LAMP. If this is the wiring configuration of your ballast/fixture, then you will instantly notice the lamp is burning very much brighter. If you measure the current, you will find your 32 watt lamp is drawing over 50 watts now. If you have success then put another ballast into the case, and wire up the other lamp. also, please be sure to visit the overdriving thread on this forum. you can probably find it by using
google "overdriving t8 garden web forum" or use the garden web forum search, or just manually search this forum.

I use a bank of HomeDepot 2 lamp shoplight fixtures. Find cheap 2 lamp fixtures, look in a variety of stores, because different suppliers might be more or less effective in the shape of the reflector, fixtures about 6" wide, with reasonable looking reflector, find yourself some ballasts that match the description on the thread (or use instant start ballasts the way I have describe above), line the reflector with mylar (as I described above), find high output cool white t8 lamps, and you're set: My contribution to the idea, is I use a dpdt toggle switch to have the option of running standard driven, and over-driven.

Here is a link that might be useful: This is where to read the discussion about overdriving.