I have got myself a back brake light and switch...now where do i put a battery and what size battery..i am not going to drill holes in my bike!!
There would appear to be plenty of room above the gearbox area or maybe under the seat of your RE2 for a small battery pack?
There shouldn't really be any need to drill any holes, as the battery holder could be fixed to a frame tube using suitable sized plastic coated or rubber covered 'P' clips, or plumbers pipe clips?
How professional it all looks in the end will of course depend to a certain extent on the inginuity of the person doing the work, the battery pack is there to do a job I suppose rather than to look original, but I see no reason why the battery pack can't be made to blend in with the appearance of the bike?
The size of the battery will be something of a compromise, as the bigger the battery, the longer it's going to last between charges (if you decide on a rechargeable type?).
I don't know how far you ride on average, but a tiny battery that will go flat during your ride isn't going to be much use. Ideally you'd probably want a battery you could recharge without removing it from the bike each time?
A 6V "Cyclon" 2.5AH or 5AH battery could be worth considering, as they are a sealed rechargeable type?
If you shop around you should be able to find them cheaper than the prices shown here?
http://www.strikalite.co.uk/prodcat_type/5/Cyclon/0/Sealed_Lead_Acid_batteries.html