I dont think I can fill the slots , it would be a dirty job and ruin the fingerboard surface. But may be can syringe epoxy , I dont know how to prevent leak from the ends. If I adhesive sellobant , it will stick to neck , , may be sanding the board edges with paper solves the problem ?
There are lots of options. Epoxy, wood veneer glued in, plastic strips... Just sand the board level after and it'll be fine. Leaving the slots will accumulate dirt and debris over time. Also, the chance of chip out when removing the frets will not be attractive.
Hand craft is not me. I will leave the slots open. Does it weaken the neck and how I can prevent warping. Guitar costs 55 dollars and luthier would be may be same.
Leaving the slots open will weaken the neck and encourage warping. Fill the slots. It isn't a difficult job. Alternately throw your $55 dollar bass in the bin or sell it to somebody else, if you are so unwilling to do any sort of work. The job is worth doing properly if at all, otherwise you will wreck your bass over time. Dare I ask how you are planning to remove the frets?
You profile doesn’t say where you are but, in the US, a hardware store, craft store of hobby shop will have wood verneer or plastic strips in the .020” range that will glue right in the slots and require minimal trimming and sanding. Worst case, wood filler could be used and the board sanded. If you can’t or won’t fill the slots perhaps you shouldn’t be pulling the frets either. Done remotely well that’s a more difficult job then filling afterward. Best of luck either way.