good heads can be found on ebay if you check the camera/tripod section.
bogan makes a very good product also and you will find lots of them there.
the fluid type heads tend to stiffen as weather gets cold.
we use the bogan 3047 head on lots of our big eye glasses. it can support
a pair of the large 80 mm spotters with no problem. you should have no problem finding a nice one used for $50 and maybe less.
the 3030 bogan is the same design but smaller scale. it will also support
a fairly large set of glasses. for a single spotter id be inclined to go with that model.
the handles on bogan models tend to be shorter than on some other makes.
and you can rearrainge them as you like. that way you dont have a handle
sticking you in the throat as you use it.
We have a fairly heavy-duty head and window pod combo. It's the Manfrotto 700 RC2 head with Manfrotto's window pod.
I assume your Slik tripod has a 3/8" threaded connector as that is what is required to connect this head to a tripod.
See attached pictures It has the 1/4" quick release plate included so you can just leave that on your spotter and quickly snap it in and out of the head.
The window pod is great for when spotting out of a vehicle, and can easily be removed when needed to be used on a tripod. The whole combo runs about $155.00 shipped to a US address.
We also, have the 494 RC2 ball heads by Manfrotto - they are lightweight and they don't have a handle like the 700 RC2 but you can basically use the spotter as the handle. Those go for about $80.00. You can also add the window pod to that as well.