For this next one I bought a cheap short 'D' handle solid steel spade, different project.
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The first posting photo shows the original Spade short white handle, then 2x red handles. One cut and the other uncut.
The spades were cheap but I found like you, the handle too short, I don't bend well.
So I looked around and found that at Mitre10 two shovels with fibre glass handles for $12ea ?, cheaper than a new long wooden handle at $30ea,
I bought those shovels and pulled the handles then attached them to the spades.
Since then I also shortened them a touch (10 inches) so they fit across the boot of the smallest car, still plenty long enough to not have to bend down, so they are comfortable to use, have leverage and look 'ferrari red' for speed.
Seriously, any cheap long handle spade or a secondhand one will do the job.
Hit up garage sales for a long handle shovel to hack the head off if you cant find something at Bunnings or another hardware.
Always seems cheaper to buy the whole thing than to just get a handle.
Those little camp, garden or kids spades are very short.
So you may find that even a stand length spade handle without the 'D' on top to be suitable, that would put the top of the handle at around waist height. It could be cheaper to start with a new standard spade at the end of the day, get a wooden handle one if you do.
You would have the shorty as a spare too.
Good luck, I will be interested in what you end up doing, let us know please.
This may be a start, but a flat spade is best because you dig straight sided plugs, "square plugs" rather than a rounded circular plug, but both work anyway.
https://www.bunnings.com.au/saxon-long-fibreglass-handle-plumbers-shovel_p0242869