A simple but effective way to use a bulkhead on an irregular surface like yours is with builders putty (many different trade names and brands) to form a flat surface. It's a putty with a catalyst hardener that you can squeeze into nooks and crannies or build up valleys. Once it sets, it can be sanded flat and paintable. Suitable for wood, metal, concrete etc. Use it on the outside so no water gets to it. There are some suitable for potable water situations if you did need to out on inside. It is a good way to plug leaks as well.