Whilst trawling my photos I came across the image below. It is the first type of internal feeder that I made; having seen the design on another web site. It was pretty easy to made, from some scrap bits of 3mm marine ply and galvanised wire. My only modification was the cut-out in the top bar that allowed access without disturbing the follower board. A 300mm hook, made from the same wire, allows the float to be lifted level with the top of the feeder box and sugar syrup gently poured over/past it. In theory it shouldn't disturb the bees very much but, bees being bees, they would often want to investigate the flowing liquid with sticky results (for them). It might be easier to use some sort of funnel arrangement rather than pouring in direct.
Anyway hope this of some help and gives you something to do during the cold winter evenings; although the clocks go forward this weekend so summer is on its way.
The small piece of wood with the hoop is a float, with three countersunk
holes, that stops bees from falling into the sugar solution