There are a couple types of insurance policies you may want to consider. The first is a ground and taxi. The second is a full flight policy.
If you buy a ground and taxi and then cancel it in a few months, you will get a short rate (i.e. not a full pro rata) return of premium.
If you buy a full flight policy, you may be wasting premium dollars while you are completing your build.
Some ground and taxi policies can be converted to a full flight policy in mid-term. When you obtain quotations, get both quotes (ground and taxi, and, full flight) from each carrier. This may impact your decision on which insurance company to select.
I found that NationAir, Jennifer Cummins, was very knowledgeable about Van's aircraft and various insurance companies.