I wouldn't mind a top box. I use the tailpack all the time, so the top box would make sense.
The only problem that I found is trying to get bits that are for my viffer. They didn't get popular until the single sided swing arm was introduced. From then on, you can get loads of parts but anything before that is a bit thin on the ground. I've bought from ebay before but the last rack that I bought, that was supposed to be for my model didn't fit, without some reworking.
I'll get it a go again. I did chase a tankbag at one point but some bugger bet me on the price.

I'll keep a look out.