Here's how I rigged a battery pack for a Wild Heerbrugg T2002. I know it's not elegant, but it works. When I bought this unit used, it had no battery pack, and I'm not necessarily going for good looks. Bottom line, I just didn't want to pay over $200 for a battery pack replacement (the only GEB68 equivalent I found online was $239).
I'm not actually using this on the field for professional purposes. I bought it for 2 reasons, 1) a practical way to teach my kids trigonometry and 2) Just because. It's a very cool piece of equipment I now own for less than $150. (My wife will never understand why. I figure, hey, this is one of my least expensive hobbies so far - she should be happy.)
Anyhow, I have a Rigid 12 volt drill, so I'm borrowing one of my batteries to power the T2002. The batteries are 12-Volt 2.0 Amp Hour Lithium-Ion Batteries. I haven't done it yet, but I could remove the lower housing of the battery (the part that snaps into the drill). If I remove this part of the battery, then I could get my T2002 battery compartment to close. For now, I'm just going to leave it open. I used very strong velcro tape to attach the battery inside the battery compartment of the T2002.
I soldered two wires to the leads inside the unit and connected the wires to two small brass pieces with shrink tubing (to insert in the battery) I obviously will need to be careful when I remove the battery to not short anything out.
If you're thinking of doing this, just about any rechargeable drill 12v battery pack could work theoretically, but most I've seen are too big to fit inside the T2002 compartment.
That's cool. Through college we are required to do "high school visits" to promote the industry and while work let me borrow one of our total stations for my last visit, I'm uncomfortable taking that much money to a school to let kids who don't respect it, around it. Your talk of using the gun to teach your kids Trig is along the lines of how I tried to tie education to surveying. I'm going to have to find an older instrument.