In order to marshal this array member you just need to flatten it to the 1500 elements that it represents
[StructLayoutAttribute(LayoutKind.Sequential)]
public struct MyStruct {
/// byte[1500]
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=1500, ArraySubType=UnmanagedType.I1)]
public byte[] myVar;
}
Do make sure to initialize the myVar
array though manually when using in C#