TBR16 has a bolt spacing of 37mm
Thats a pain, you have couple of options but they still require a bit of tweaking if you use this one http://www.aluminium-profile.co.uk/acatalog/40x80.pdf then the 40mm spacing with the 10mm slot gives you 30mm edge to edge which should be just enough to get the bolts in.

The tricky bit is the fasteners, most have a rebate on the edges so they centre in the slot which is no good. if you look here http://catalog.item.info/Onlinekatal...00020610/37344

The nuts are just flat bar with cut corners and m5 thread, this means you can move them over a bit to suit your spacing. Ideally you could make your own with offset holes that fit the 10mm slot.

Item also do a 12mm slot version that might work with the offset idea.

Another option is to remove the bolts that hold the rail and support together and replace with longer ones drilled all the way throught the profile, I have some 16mm rails mounted like this but using soilid Ali square section, very strong.

As always there are lots of ways of acheiving the same goal but ulimatley if you want easy go with the hiwin profile rails, if you want cheaper then I'm afraid it means improvising a bit