They work well for all of my 1.25" eyepieces and will barely fit around my large 2" eyepieces (Naglers). The 2" heaters will fit around most other 2" eyepieces with no trouble.? If you view mostly with large Naglers, you may want to consider using 3" heaters instead.
The Telrad heater is sort of a rectangle shape.? When you look at it, you may ask, "How does this thing attach?".? It's very simple.? You put it on the side of your Telrad, where it fogs up, and secure a rubber band around it and the telrad.? The rubber band sometimes gets in the way of your switch, but it is a minor inconvenience.
For a look at how these things appear when setup, take a look at my observatory setup in my observatory pages.
Kendrick also makes a fine battery pack.? I consider them somewhat expensive.? Power is power and a sealed battery is pretty much the same wherever you go.? Wallmart and Costco sell emergency battery jump kits for around 60 to 80 dollars.? I purchased mine at Costco.? It has a 34 amp hour battery, and emergency light, nifty LED's that give charge status, two 12 volt sockets, and cables to help start my car in an emergency.? The Kendrick 17 hour pack comes in a very nice Cordura pack and is somewhat compact.? Costco... $57, Kendrick $109.? Take your pick.
The cheaper route? Chemical heaters
Okay, you don't want to spring for the Kendrick system. The costs for all those heaters adds up.? A controller, battery pack, power source for recharging.... it can get to be a real hassle.