Watch out with DIY bra/belts - IMO unless you spend a lot of time and effort (and maybe money
) you can end up looking waaaaay more amateur than you would in a nice skirt, hipscarf and choli. Lingerie bras MUST properly covered and decorated, or they have a tendency to look exactly what they are - underwear. Stiff belts need to be shaped for a good fit, if they have any width to them.
I'd go for a more informal look (especially as it's for friends) - maybe a circle type skirt, colourful hip scarf and/or coin belt and a top. You can go really opulent with luxurious fabrics and layers. Then glitz it up with accessories and fab makeup to get the full-on exotic belly dancer effect!
Shira has some good suggestions and links for costuming.
Belly Dancing: Costume Corner
And there's a downloadable booklet here:
Davina aka Dawn Devine Brown - Author - Costumer - Dancer - Historian
Have fun!