Just weld it....i welded an entire sill on after not having done it in over 10 years since i was an apprentice and was easy. Just be stitch welding so you can't really go wrong if you work with a pro to keep you in check.
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Your under selling yourself mate, that weld job you did looks tidy even for someone who does it everyday.
Most folks with basic welding skill will blow most of the metal away putting a sill in.
What cost?
My new sills cost me £37 plus £150 labour.
Sills are cheap but a £150 each or for the pair? and properly sealed and painted in?
Full cut out and weld in or weld over the top?