Finally got my FT5 printing this week, probably the longest build of an FT5 ever (got the kit September last year!)..
Went with 0.8mm thick PEI from ebay and just clamped on top of mirror for starters, think I'm only ever going to use PLA as ventilation is a problem here - but PLA on PEI seems to work alright even without a heated bed installed, hopefully a fine sand will stop it curling at the edges on larger prints. Didn't bother soldering the heated bed because I figured it'll just warp, but definitely going to replace with a silicon heated mat at some point in the future.
For mirrors, Ikea Lots 4 pack of mirrors is quite cheap and works pretty great, they're protective plastic backed so you definitely want to peel that off first!
$8.99 for most of you guys, $14.95 here in Aus - which isn't too bad.